1. Anatomy and Physiology of the Skin
The skin is the largest organ in the body consisting of several layers and plays an important role in biologic homeostasis, and is comprised of the epidermis and the dermis. The epidermis, which is composed of several layers beginning with the stratum corneum, is the outermost layer of the skin, and the innermost skin layer is the deep dermis. The skin has multiple functions, including thermal regulation, metabolic function (vitamin D metabolism), and immune functions. FIG. 1 presents a diagram of skin anatomy.
In humans, the usual thickness of the skin is from 1-2 mm, although there is considerable variation in different parts of the body. The relative proportions of the epidermis and dermis also vary, and a thick skin is found in regions where there is a thickening of either or both layers. For example, on the interscapular (between the shoulder blades) region of the back, where the dermis is particularly thick, the skin may be more than 5 mm thick, whereas on the eyelids it may be less than 0.5 mm. Generally, the skin is thicker on the dorsal or extensor surfaces of the body than on the ventral or flexor surfaces; however, this is not the case for the hands and feet. The skin of the palms and soles is thicker than on any dorsal surface except the intrascapular region. The palms and soles have a characteristically thickened epidermis, in addition to a thick dermis.
The entire skin surface is traversed by numerous fine furrows, which run in definite directions and cross each other to bound small rhomboid or rectangular fields. These furrows correspond to similar ones on the surface of the dermis so that, in section, the boundary line between epidermis and dermis appears wavy. On the thick skin of the palms and soles, the fields form long, narrow ridges separated by parallel coursing furrows, and in the fingertips these ridges are arranged in the complicated loops, whorls (verticil) and spirals that give the fingerprints characteristic for each individual. These ridges are more prominent in those regions where the epidermis is thickest.
Where there is an epidermal ridge externally there is a corresponding narrower projection, called a “rete peg,” on the dermal surface. Dermal papillae on either side of each rete peg project irregularly into the epidermis. In the palms and soles, and other sensitive parts of the skin, the dermal papillae are numerous, tall and often branched, and vary in height (from 0.05 mm to 0.2 mm). Where mechanical demands are slight and the epidermis is thinner, such as on the abdomen and face, the papillae are low and fewer in number.
Epidermis
The epidermis provides the body's buffer zone against the environment. It provides protection from trauma, excludes toxins and microbial organisms, and provides a semi-permeable membrane, keeping vital body fluids within the protective envelope. Traditionally, the epidermis has been divided into several layers, of which two represent the most significant ones physiologically. The basal-cell layer, or germinative layer, is of importance because it is the primary source of regenerative cells. In the process of wound healing, this is the area that undergoes mitosis in most instances. The upper epidermis, including stratum and granular layer, is the other area of formation of the normal epidermal-barrier function.
Stratum Corneum and the Acid Mantle
Stratum corneum is an avascular, multilayer structure that functions as a barrier to the environment and prevents transepidermal water loss. Recent studies have shown that enzymatic activity is involved in the formation of an acid mantle in the stratum corneum. Together, the acid mantle and stratum corneum make the skin less permeable to water and other polar compounds, and indirectly protect the skin from invasion by microorganisms. Normal surface skin pH is between 4 and 6.5 in healthy people; it varies according to area of skin on the body. This low pH forms an acid mantle that enhances the skin barrier function.
Other Layers of the Epidermis
Other layers of the epidermis below the stratum corneum include the stratum lucidum, stratum granulosum, stratum germinativum, and stratum basale. Each contains living cells with specialized functions (FIG. 2). For example melanin, which is produced by melanocytes in the epidermis, is responsible for the color of the skin. Langerhans cells are involved in immune processing.
Dermal Appendages
Dermal appendages, which include hair follicles, sebaceous and sweat glands, fingernails, and toenails, originate in the epidermis and protrude into the dermis hair follicles and sebaceous and sweat glands contribute epithelial cells for rapid reepithelialization of wounds that do not penetrate through the dermis (termed partial-thickness wounds). The sebaceous glands are responsible for secretions that lubricate the skin, keeping it soft and flexible. They are most numerous in the face and sparse in the palm of the hands and soles of the feet. Sweat gland secretions control skin pH to prevent dermal infections. The sweat glands, dermal blood vessels, and small muscles in the skin (responsible for goose pimples) control temperature on the surface of the body. Nerve endings in the skin include receptors for pain, touch, heat, and cold. Loss of these nerve endings increases the risk for skin breakdown by decreasing the tolerance of the tissue to external forces.
The basement membrane both separates and connects the epidermis and dermis. When epidermal cells in the basement membrane divide, one cell remains, and the other migrates through the granular layer to the surface stratum corneum. At the surface, the cell dies and forms keratin. Dry keratin on the surface is called scale. Hyperkeratosis (thickened layers of keratin) is found often on the heels and indicates loss of sebaceous gland and sweat gland functions if the patient is diabetic. The basement membrane atrophies with aging; separation between the basement membrane and dermis is one cause for skin tears in the elderly.
Dermis
The dermis, or the true skin, is a vascular structure that supports and nourishes the epidermis. In addition, there are sensory nerve endings in the dermis that transmit signals regarding pain, pressure, heat, and cold. The dermis is divided into two layers: the superficial dermis and the deep dermis.
The superficial dermis consists of extracellular matrix (collagen, elastin, and ground substances) and contains blood vessels, lymphatics, epithelial cells, connective tissue, muscle, fat, and nerve tissue. The vascular supply of the dermis is responsible for nourishing the epidermis and regulating body temperature. Fibroblasts are responsible for producing the collagen and elastin components of the skin that give it turgor. Fibronectin and hyaluronic acid are secreted by the fibroblasts. The structural integrity of the dermis plays a role in the normal function and youthful appearance of the skin.
The deep dermis is located over the subcutaneous fat; it contains larger networks of blood vessels and collagen fibers to provide tensile strength. It also consists of fibroelastic connective tissue, which is yellow and composed mainly of collagen. Fibroblasts are also present in this tissue layer. The well-vascularized dermis withstands pressure for longer periods of time than subcutaneous tissue or muscle. The collagen in the skin gives the skin its toughness. Dermal wounds, e.g., cracks or pustules, involve the epidermis, basal membrane, and dermis. Typically, dermal injuries heal rapidly.
2. Effects of Application to the Skin
Substances are applied to the skin to elicit one or more of four general effects: an effect on the skin surface, an effect within the stratum corneum; an effect requiring penetration into the epidermis and dermis; or a systemic effect resulting from delivery of sufficient amounts of a given substance through the epidermis and the dermis to the vasculature to produce therapeutic systemic concentrations. One example of an effect on the skin surface is formation of a film. Film formation may be protective (e.g., sunscreen) and/or occlusive (e.g., to provide a moisturizing effect by diminishing loss of moisture from the skin surface). One example of an effect within the stratum corneum is skin moisturization; which may involve the hydration of dry outer cells by surface films or the intercalation of water in the lipid-rich intercellular laminae; the stratum corneum also may serve as a reservoir phase or depot wherein topically applied substances accumulate due to partitioning into, or binding with, skin components.
It generally is recognized that short-term penetration occurs through the hair follicles and the sebaceous apparatus of the skin, while long term penetration occurs across cells. Penetration of a substance into the viable epidermis and dermis may be difficult to achieve, but once it has occurred, the continued diffusion of the substance into the dermis is likely to result in its transfer into the microcirculation of the dermis and then into the general circulation. It is possible, however, to formulate delivery systems that provide substantial localized delivery.
Percutaneous absorption is the absorption of substances from outside the skin to positions beneath the skin, including into the blood stream. The epidermis of human skin is highly relevant to absorption rates. Passage through the stratum corneum marks the rate-limiting step for percutaneous absorption. The major steps involved in percutaneous absorption of, for example, a drug, include the establishment of a concentration gradient, which provides a driving force for drug movement across the skin, the release of drug from the vehicle into the skin-partition coefficient and drug diffusion across the layers of the skin-diffusion coefficient. The relationship of these factors to one another is summarized by the following equation:J=Cveh×Km·D/x  [Formula 1]
where J=rate of absorption; Cveh=concentration of drug in vehicle; Km=partition coefficient; and D=diffusion coefficient.
The many factors that affect the rate of percutaneous absorption of a substance include, without limitation, the following: (i) Concentration. The more concentrated the substance, the greater the absorption rate. (ii) Size of skin surface area. The wider the contact area of the skin to which the substance is applied, the greater the absorption rate. (iii) Anatomical site of application. Skin varies in thickness in different areas of the body. A thicker and more intact stratum corneum decreases the rate of absorbency of a substance. The stratum corneum of the facial area is much thinner than, for example, the skin of the palms of the hands. The facial skin's construction and the thinness of the stratum corneum provide an area of the body that is optimized for percutaneous absorption to allow delivery of active agents both locally and systemically through the body. (iv) Hydration. Hydration (meaning increasing the water content of the skin) causes the stratum corneum to swell which increases permeability. (v) Skin temperature. Increased skin temperature increases permeability. (vi) Composition. The composition of the compound and of the vehicle also determines the absorbency of a substance.
Most substances applied topically are incorporated into bases or vehicles. The vehicle chosen for a topical application will greatly influence absorption, and may itself have a beneficial effect on the skin. Factors that determine the choice of vehicle and the transfer rate across the skin are the substance's partition coefficient, molecular weight and water solubility. The protein portion of the stratum corneum is most permeable to water soluble substances and the lipid portion of the stratum corneum is most permeable to lipid soluble substances. It follows that substances having both lipid and aqueous solubility may traverse the stratum corneum more readily. (See Dermal Exposure Assessment: Principles and Applications, EPA/600/8-91/011b, January 1992, Interim Report—Exposure Assessment Group, Office of Health and Environmental Assessment, U.S. Environmental Protection Agency, Washington, D.C. 20460).
3. Wound Healing
The term “wound healing” refers to the process by which the body repairs trauma to any of its tissues, especially those caused by physical means and with interruption of continuity. The term “wound healing agent” refers to any substance that facilitates the wound healing process.
A wound-healing response often is described as having three distinct phases-injury, inflammation and repair. Generally speaking, the body responds to injury with an inflammatory response, which is crucial to maintaining the health and integrity of an organism. If however it goes awry, it can result in tissue destruction.
Phase I: Injury
Injury caused by factors including, but not limited to, autoimmune or allergic reactions, environmental particulates, infection or mechanical damage often results in the disruption of normal tissue architecture, initiating a healing response. Damaged epithelial and endothelial cells must be replaced to maintain barrier function and integrity and prevent blood loss, respectively. Acute damage to endothelial cells leads to the release of inflammatory mediators and initiation of an anti-fibrinolytic coagulation cascade, temporarily plugging the damaged vessel with a platelet and fibrin-rich clot.
Platelet recruitment, degranulation and clot formation rapidly progress into a phase of vasoconstriction with increased permeability, allowing the extravasation (movement of white blood cells from the capillaries to the tissues surrounding them) and direct recruitment of leukocytes to the injured site. The basement membrane, which forms the extracellular matrix underlying the epithelium and endothelium of parenchymal tissue, precludes direct access to the damaged tissue. To disrupt this physical barrier, zinc-dependent endopeptidases, also called matrix metalloproteinases (MMPs), cleave one or more extracellular matrix constituents allowing extravasation of cells into, and out of, damaged sites. Specifically, MMP-2 (gelatinase A, Type N collagenase) and MMP-9 (gelatinase B, Type IV collagenase) cleave type N collagens and gelatin, two important constituents of the basement membrane. Recent studies have found that MMP-2 and MMP-9 are upregulated, highlighting that tissue-destructive and regenerative processes are common in fibrotic conditions. The activities of MMPs are controlled by several mechanisms including transcriptional regulation, proenzyme regulation, and specific tissue inhibitors of MMPs. The balance between MMPs and the various inhibitory mechanisms can regulate inflammation and determine the net amount of collagen deposited during the healing response.
Phase II: Inflammation
Once access to the site of tissue damage has been achieved, chemokine gradients recruit inflammatory cells. Neutrophils, eosinophils, lymphocytes, and macrophages are observed at sites of acute injury with cell debris and areas of necrosis cleared by phagocytes.
The early recruitment of eosinophils, neutrophils, lymphocytes, and macrophages providing inflammatory cytokines and chemokines can contribute to local TGF-β and IL-13 accumulation. Following the initial insult and wave of inflammatory cells, a late-stage recruitment of inflammatory cells may assist in phagocytosis, in clearing cell debris, and in controlling excessive cellular proliferation, which together may contribute to normal healing. Late-stage inflammation may serve an anti-fibrotic role and may be required for successful resolution of wound-healing responses. For example, a late-phase inflammatory profile rich in phagocytic macrophages, assisting in fibroblast clearance, in addition to IL-10-secreting regulatory T cells, suppressing local chemokine production and TGF-β, may prevent excessive fibroblast activation.
The nature of the insult or causative agent often dictates the character of the ensuing inflammatory response. For example, exogenous stimuli like pathogen-associated molecular patterns (PAMPs) are recognized by pathogen recognition receptors, such as toll-like receptors and NOD-like receptors (cytoplasmic proteins that have a variety of functions in regulation of inflammatory and apoptotic responses), and influence the response of innate cells to invading pathogens. Endogenous danger signals also can influence local innate cells and orchestrate the inflammatory cascade.
The nature of the inflammatory response dramatically influences resident tissue cells and the ensuing inflammatory cells. Inflammatory cells themselves also propagate further inflammation through the secretion of chemokines, cytokines, and growth factors. Many cytokines are involved throughout a wound-healing and fibrotic response, with specific groups of genes activated in various conditions.
Phase III: Tissue Repair and Contraction
The closing phase of wound healing consists of an orchestrated cellular re-organization guided by a fibrin (a fibrous protein that is polymerized to form a “mesh” that forms a clot over a wound site)-rich scaffold formation, wound contraction, closure and re-epithelialization. The vast majority of studies elucidating the processes involved in this phase of wound repair have come from dermal wound studies and in vitro systems.
Myofibroblast-derived collagens and smooth muscle actin (α-SMA) form a provisional extracellular matrix, with macrophage, platelet, and fibroblast-derived fibronectin forming a fibrin scaffold. Collectively, these structures are commonly referred to as granulation tissues.
In addition to fibronectin, the provisional extracellular matrix consists of glycoproteins (such as PDGF), glycosaminoglycans (such as hyaluronic acid), proteoglycans and elastin. Growth factor and TGF-β-activated fibroblasts migrate along the extracellular matrix network and repair the wound. Within skin wounds, TGF-β also induces a contractile response, regulating the orientation of collagen fibers. Fibroblast to myofibroblast differentiation, as discussed above, also creates stress fibers and the neo-expression of α-SMA, both of which confer the high contractile activity within myofibroblasts. The attachment of myofibroblasts to the extracellular matrix at specialized sites called the “fibronexus” or “super mature focal adhesions” pull the wound together, reducing the size of the lesion during the contraction phase. The extent of extracellular matrix laid down and the quantity of activated myofibroblasts determines the amount of collagen deposition. To this end, the balance of matrix metalloproteinases (MMPs) to tissue inhibitor of metalloproteinases (TIMPs) and collagens to collagenases vary throughout the response, shifting from pro-synthesis and increased collagen deposition towards a controlled balance, with no net increase in collagen. For successful wound healing, this balance often occurs when fibroblasts undergo apoptosis, inflammation begins to subside, and granulation tissue recedes, leaving a collagen-rich lesion. From skin studies, re-epithelialization of the wound site re-establishes the barrier function and allows encapsulated cellular re-organization. Several in vitro and in vivo models, using human or rat epithelial cells grown over a collagen matrix, or tracheal wounds in vivo, have been used to identify significant stages of cell migration, proliferation, and cell spreading. Rapid and dynamic motility and proliferation, with epithelial restitution from the edges of the denuded area occur within hours of the initial wound. In addition, sliding sheets of epithelial cells can migrate over the injured area assisting wound coverage. Several factors have been shown to regulate re-epithelialization, including serum-derived transforming growth factor alpha (TGF-α), and matrix metalloproteinase-7 (MMP-7) (which itself is regulated by TIMP-1).
4. Delivery Systems for Topical Administration
Many active agents are administrated enterally or parenterally. Enteral routes of administration involve administration to any part of the gastrointestinal tract, typically via oral forms, e.g., pills, tablets, emulsions, and syrups, or via rectal forms, e.g., enemas, Murphy drips, and suppositories. Parenteral routes of administration involve administration by some means other than oral or rectal, typically via injection. While such administration routes allow for accurate and consistent dosing, such routes necessarily yield systemic effects, e.g., vestibular symptoms, headache and general malaise, and gastrointestinal symptoms, which in certain circumstances are not desirable.
Topical routes of administration involve administration to a body surface, such as the skin, or mucous membranes. Many forms of topical administration involve applying a therapeutic agent directly to the skin; inhalable mediations, eye drops, and ear-drops also are considered topical administration forms. Although topical administration generally provides a local effect, many topically administered drugs likewise can exhibit systemic effects, such as vestibular symptoms (e.g., vertigo, dizziness or blurred vision), headache and general malaise, gastro-intestinal symptoms, such as diarrhea, nausea, gas, cramps, dry nose and dry mouth.
Formulations for topical application can take the compositional form of a liquid, a semisolid dosage form (e.g., a paste, a cream, a lotion, a powder, an ointment or a gel) or a patch.
Liquid formulations do not readily stay in place and can be messy. Semisolid formulations offer some advantages characteristic of topical administration, such as ease of application, and increased local doses of active agent, with reduced systemic effects, but their potential disadvantages include the need for repeated application, difficulties in accurate dosing, and messy or unattractive cosmetic attributes, all of which can lead to poor user compliance, and unintentional removal or transfer of active agent via contact with objects or other persons.
Topical patches, which are available in multiple forms including single and multi-layer drug-in-adhesive forms, matrix forms, and reservoir forms, address several of the shortcomings of semisolid formulations, for example, reducing the need for repeated application, providing accurate, and controlled release of active agent, and reducing the likelihood of unintentional removal or transfer of drug or active agent via contact with objects or other persons, but have a finite size and shape. Because topical patches have a finite size and shape, the application area is determined by the dimensions of the patch rather than the dimensions of the affected site. Accordingly, it may be necessary to use a number of patches in order to cover a large affected site. Furthermore, topical patches typically lack sufficient flexibility to be effectively administered to joints or other areas of skin subject to significant stretching movements. Topical patches can also lead to user discomfort, particular in warmer climates, and can be aesthetically unpleasing, which can also lead to poor user compliance.
A number of attempts have been made for delivering therapeutic formulations topically. One common problem inherent to topical formulations that has been experienced thus far has been to control the therapeutic active agent, as well as the other composition components, such that they are specifically confined in the area of the skin in which they have been directly applied. This is turn may result in too fast release of the drug with the consequence of causing undesirable spikes and high levels of the drug in the bloodstream, thereby creating deleterious effects such as e.g. unwanted side-effects, wash-out or metabolism of the drug.
The described invention addresses and overcomes these shortcomings. The described composition and method provides a safe and effective topical therapeutic drug delivery platform that can deliver drugs locally into the skin. The described invention is effective to deliver the components of the pharmaceutical formulation into the skin, to keep them in the skin, and to reduce the potential of the active therapeutic agent or its metabolites to enter the bloodstream. Consequently, the active therapeutic agent executes its effective biological function locally at the tissue of interest once being released from the skin.